Process for the production of electroluminescent silicon structures

ABSTRACT

A process for the production of electroluminescent silicon structures, including: placing a silicon wafer in an acid bath; anodizing the silicon wafer in the acid bath using the apparatus of FIG. 2; illuminating the anode side of the silicon wafer during at least part of the time the silicon wafer is being placed in the acid bath and is being anodized; causing at least some areas of the monocrystalline silicon of the silicon wafer to be converted into a microporous silicon layer; and forming two contacts by means of which a voltage can be applied to the microporous silicon layer.

FIELD OF THE INVENTION

The present invention refers to a process for the production ofelectroluminescent silicon structures.

DESCRIPTION OF THE PRIOR ART

For a long time, it has been taken for granted that the only structuressuitable for light emission are structures consisting of semiconductormaterials which have direct band transition. As is generally known, theband gap of semiconductor material is the difference between the energylevels of the valence band and of the conduction band, which is filledwith electrons. In semiconductor materials having a direct bandtransition, the highest energetic state in the valence band liesdirectly below the lowest energetic state of the conduction band. Thishas the effect that, when a direct transition of electrons into thevalence band takes place, a recombination of these electrons with holes(positive charge carriers) will occur, whereby photons will be producerwhose energy corresponds to the band gap of the semiconductor material.

Typical materials having such a direct band transition are e.g. GaAscompound semiconductors, and, consequently, such GaAs compoundsemiconductors are frequently used for producing light-emittingelements.

In contrast to GaAs, silicon is a semiconductor material having anindirect band transition. In such materials, the highest energetic statein the valence band is displaced relative to the lowest energetic statein the conduction band so that electrons cannot directly drop into thevalence band. In order to achieve suitable energy levels, the electronshave to combine with holes as well as with phonons in such materialshaving an indirect band transition. The likelihood that this processtakes place is, in view of the fact that three particles participate,very small.

Lately, it has been found out that, notwithstanding the fact thatsilicon is a semiconductor material having an indirect band transition,semiconductor structures consisting of silicon are suitable forphotoluminescence provided that the silicon is anodized in an aqueoushydrofluoric acid bath so as to produce microporous silicon layers.

By way of example, reference is made to L. T. Canham, Appl. Phys. Lett.57 (10), Sep. 3, 1990, pages 1046 to 1048. Within the microporous layershaving pore sizes of less than 2 nm (20 A), the electron movement islimited to one dimension, i.e. to a direction of movement along the socalled "quantum conductors" or "quantum wires" which are defined betweenthe pores. By limiting the possibilities of movement of the electrons,these quantum conductors effect a direct transition of the electronsbetween the conduction band and the valiance band. In other words, theband structure is purposefully influenced by means of a local limitationof the possibilities of movement of the electrons. As has, however, beenexplained by Canham in the cited publication (cf. page 1047F rightcolumn, last paragraph), the luminescence of silicon only occurs in thecase of newly anodized silicon substrates. In other words, the attemptto maintain a stable photoluminescence of such silicon structures hasnot been successful up to now.

Also the publication "Silicon Lights Up", Scientific American, July1991, pages 86 and 87, discloses that porous silicon structures, whichare produced in an aqueous hydrofluoric acid making use of a siliconwafer, are adapted to be excited by means of light such thatphotoluminescence occurs.

Furthermore, reference is made to the fact that it was successfullyattempted to electrically excite the luminescence in silicon structures;however, neither the nature of the structure nor the production processof said structure are disclosed.

The technical publication V. Lehmann, Appl. Phys. Lett. 58 (8), Feb. 25,1991, pages 856 to 858, discloses that, in porous silicon layers, whichare produced by means of anodization in an hydrofluoric acidelectrolyte, two-dimensional quantum concentrations or quantum wires andquantum conductors, respectively, are produced, which cause a change inthe energetic band gap of the microporous silicon structures incomparison with monocrystalline silicon.

The publication C. Pickering, J. Phys. C: Solid State Phys. 17 (1981),pages 6535 to 6552, deals with optical studies of porous silicon filmstructures, which have been produced by an anodization of silicon wafersin an aqueous solution of hydrofluoric acid. As has been explained onpage 6537, second section, last paragraph, photoluminescencemeasurements have, however, been carried out only at very lowtemperatures of 4.2 K.

The publication of H. Foll, Appl. Phys. A 53, 1991, pages 8 to 19, dealswith the properties of silicon-electrolyte junctions of silicon sampleswhich are immersed in an aqueous hydrofluoric acid solution, and theformation of porous silicon layers is disclosed in this publication aswell. Said publication is, however, not concerned with photoluminescentor electroluminescent silicon structures, but with the examination ofthe properties of the silicon-hydrofluoric acid junction. For thispurpose, a universal electrochemical silicon analyzer is shown, in whichthe front of the wafer is illuminated with a laser beam having awavelength of small penetration depth so as to define the diffusionlength and the surface recombination rate on a silicon wafer. Minoritycarriers are thus generated close to the front surface. The resultantrear photocurrent is measured. It follows that this publication does notdeal with luminescent properties of silicon.

U.S. Pat. No. 4,092,445 describes a process for the production ofmicroporous silicon structures, said process comprising the steps ofplacing a silicon wafer in an acid bath, anodizing the silicon wafer insaid acid bath, and illuminating the silicon wafer while said siliconwafer is being placed in the acid bath and is being anodized, thuscausing at least some areas of the mono-crystalline silicon of thesilicon wafer to be converted into a microporous silicon layer.

OBJECT OF THE INVENTION

It is the object of the present invention to provide a process for theproduction of electroluminescent silicon structures.

SUMMARY OF THE INVENTION

This object is achieved by the following process steps:

placing a silicon wafer in an acid bath;

anodizing the silicon wafer in said acid bath;

illuminating the anode side of said silicon wafer during at least partof the time the silicon wafer is in the acid bath and being anodized,thus causing at least some areas of the monocrystalline silicon of thesilicon wafer to be converted into a microporous silicon layer; and

forming a first and a second contact by means of which a voltage can beapplied to the microporous silicon layer.

The present invention is based on the finding that the process for theproduction of electroluminescent silicon structures, which is known fromthe prior art and which includes the steps of placing a silicon wafer inan acid bath and anodizing it in said bath so as to produce amicroporous silicon layer, can be adapted and modified for theproduction of electroluminescent silicon structures by illuminating theanode side of the silicon wafer during at least part of the time saidsilicon wafer is placed in the acid bath and being anodized, whereupontwo contacts are formed by means of which a voltage can be applied tothe microporous silicon layer.

Making reference to the appended figures, the production processaccording to the present invention, a device suitable for carrying outsaid production process as well as an electroluminescent siliconstructure produced in accordance with the production process accordingto the present invention are explained in detail hereinbelow.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional representation of an electroluminescentsilicon structure produced in accordance with the production processaccording to the present invention; and

FIG. 2 shows a device for carrying out essential steps of the productionprocess according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Making reference to FIG. 1, the structure of an electro luminescentsilicon component, which has been manufactured in accordance with theproduction process according to the present invention, will be explainedbefore the sequence of process steps is described.

The silicon component, as shown in FIG. 1 and which is designatedgenerally by reference numeral 1, is obtained by separation of aplurality of identical silicon components formed on a common siliconwafer.

The silicon component 1 comprises a p-type substrate 2 having arrangedthereon an oxide layer 3 with a central recess 4, which, in turn, iscovered by a nitride layer 5. Nitride layer 5 has provided thereon ametallic coating 6 in the form of a chromium-gold alloy extending e.g.annularly round the recess 4. Below said recess 4, the p-type substrate2 is followed by a p+ doping region 7 enclosing in a troughlike manneran n+ doping region, which is located directly below the recess 4, so asto form a p-n junction 9. The silicon within the n+ doping region 8 aswell as within part of the p+ doping region 7 is a microporous siliconlayer 10.

In other words, the p-n junction 9 lies within the microporous siliconlayer 10. An additional nitride layer 11 covers the front of thecomponent 1 with the exception of the recess 4 in the area of themicroporous silicon layer 10 and an additional recess 12 in the area ofthe metallic coating 6.

The front of the silicon component 1 is covered throughout the wholearea thereof with a transparent or at least partially light-transmittingfirst electrode 13, which can be formed e.g. by a gold contact layerhaving a thickness of 120 nm or by an indium tin oxide layer having athickness of approx. 200 nm.

The back of the p-type substrate 2 is provided with a second electrode14 in the form of an ohmic contact.

The production process of this silicon component 1 comprises thefollowing process steps: a silicon wafer has applied thereto the siliconoxide layer 2 which serves as an implantation mask, said silicon oxidelayer 2 being applied e.g. by thermal oxidation. The doping of thesilicon wafer is preferably chosen such that the resistivity thereof isbetween 1 and 10 Ohm cm. Subsequently, the silicon oxide layer 2 isstructured by known photolithographic measures. Using this implantationmask, a first doping step is now carried out for p+-doping the lowerdoping region, whereupon n+-doping of a doping region located above thelower doping region is effected with reduced implantation energy, adrive-in diffusion process being then carried out for driving in thedopants. This step is followed by full-area deposition of the nitridelayer 5, which has then applied thereto a chromium-gold metallic coating6 throughout the whole area thereof; this chromium-gold metallic coating6 is then structured using measures which are known and which are takenfor producing the annularly extending contact zone 6. Now, full-areadeposition of the additional nitride layer 11 is carried out, whereupona photoresist (not shown) is applied and structured. After adequatephotolithographic steps, the nitride layer 5, 11 is etched away in thearea of the central recess 4 as well as in the area of the additionalrecess 12 above the metallic coating, whereupon the photoresist isremoved.

The anodization region of the silicon wafer 20 can be delimitedlaterally by an acid-resistant masking layer, said acid-resistantmasking layer being preferably light-proof.

A silicon wafer 20 comprising a plurality of the silicon components 1structured by the process steps which have been explained hereinbeforeare now further treated in a production device 21 used for carrying outthe essential steps of the production process according to the presentinvention which will be explained hereinbelow.

The production device 21 includes a basin 22 which contains an acid bath23 comprising 2 to 50 percent by weight of hydrofluoric acid, theresidue being ethanol and water.

The acid bath 23 has provided therein an anode 24 as well as a cathode25, which are arranged in opposite, spaced relationship.

A holding device 26 has, at its periphery, a structural design of such anature that it is in sealing contact with the walls of the basin 22filled with acid, when it has been inserted into said basin 22 fromabove. The holding device 26 is provided with a central recess 27, thesilicon wafer 20 being held at the location of said recess 27 in such away that the edge portions thereof are sealingly enclosed.

Hence, the holding device is arranged in such a way that a flow ofcurrent between the anode 24 and the cathode 25 must pass through thesilicon wafer 20 vertically to the main surfaces thereof.

An illumination device 28 in the form of a mercury lamp or a halogenlamp is arranged above the acid bath 23, or, in cases in which anacid-resistant illumination device 28 is used, within the acid bath 23,in such a way that the silicon wafer 20 is illuminated from its anodeside. If the illumination device 28 is arranged outside of the basin 22filled with acid, said basin 22 is preferably provided with a window(not shown) which allows a passage of light. In addition, when theillumination device 28 is arranged above the basin 22 filled with acid,a mirror can be provided in the acid bath 23, said mirror being used fordeflecting the rays towards the wafer.

The illumination device can also be a laser. The laser which ispreferably used in this case is an argon-ion laser having a wavelengthof 488 nm and a power density per unit area of 5 W/cm2.

In this case, it is possible to provide selectively luminescent areas byselective anodizing.

The continued production process-of the silicon components 1 within thesilicon wafer 20, which have been produced by the process stepsdescribed at the beginning, will now be explained in detail again withreference to FIG. 2.

When the silicon wafer 20 has been inserted in the holding device 26,said holding device is introduced into the basin 22 from above. Now thesilicon wafer is anodized with a current density of from 2 to 500 mA/cm2by applying an adequate direct current to the anode 24 and the cathode25; in the course of this process, the silicon wafer 20 is subjected toa conversion of the monocrystalline silicon into a microporous,electroluminescent silicon layer 10 in the area of the recesses 4 of thesilicon components 1. The anodizing process in the acid bath as well asthe illumination by means of the illumination device 28 are carried outfor such a period of time that the microporous silicon layer 10 willextend into the substrate 2 up to a point beyond the p-n junction 9.Typical anodizing and illumination periods are between 10 seconds and 20minutes.

After rinsing of the silicon component, said silicon component isprovided with the ohmic contact 14 at the back as well as with thetransparent electrode 13 at the front thereof. The electrode at thefront can be realized by applying a gold contact having a thickness of120 nm or by applying an indium tin oxide layer having a thickness of200 nm.

When the silicon components 1 have been separated by means of anadequate division of the silicon wafer, the component is finished, withthe exception of a housing which has to be provided.

In the preferred embodiment, the silicon component 1 produced inaccordance with the process according to the present invention has a p-njunction within the porous silicon layer 10. Although such a p-njunction is regarded as being a preferred feature which serves thepurpose of increasing the quantum efficiency, the p-n junction is notnecessary for the fundamental operability of the component such that then+ doping region 8 can be dispensed with.

It is also possible to use doping polarities which are opposite to thoseused in connection with the present embodiment.

Furthermore, it is not necessary that the second electrode 14 be formedas a rear ohmic contact at the back of the substrate 2. Any kind ofcontact making with the substrate 2, which may just as well beimplemented at the front, is possible.

When the rear contact is omitted, it is also possible to form contactsin an interdigital structure on the microporous layer by means of theupper metallic coating.

What is claimed is:
 1. A process for the production ofelectroluminescent silicon structures, comprising the following processsteps:placing a monocrystalline silicon wafer in an acid bath; anodizingthe silicon wafer in said acid bath; illuminating the anode side of saidsilicon wafer during at least part of the time the silicon wafer isbeing placed in the acid bath and is being anodized, thus causing atleast some areas of the monocrystalline silicon of the silicon wafer tobe converted into a microporous silicon layer; and forming a first and asecond contact by means of which a voltage can be applied to themicroporous silicon layer.
 2. A process according to claim 1, whereinthe anodizing is carried out with a current density of from 2 to 500mA/cm².
 3. A process according to claim 1, wherein the acid bathcomprises 2 to 50 percent by weight of hydrofluoric acid and that theresidual thereof is ethanol and water.
 4. A process according to claim1, wherein the silicon wafer is n doped.
 5. A process according to claim4, wherein the doping of the silicon wafer is chosen such that theresistivity thereof is between 1 and 10 Ohm cm.
 6. A process accordingto claim 1, wherein the illumination is effected by means of a mercurylamp or a halogen lamp or a laser.
 7. A process according to claim 1,comprising the following initial process steps carried out before thesilicon wafer is placed in the acid bath:producing a silicon oxide layerwhich serves as an implantation mask; structuring the silicon oxidelayer; implanting dopants with the same conductivity type as the siliconwafer; driving in the dopants in a drive-in diffusion process;depositing a nitride layer in a full-area deposition mode; applying achromium-gold metallic coating; structuring the chromium-gold metalliccoating; depositing an additional nitride layer; applying andstructuring a photoresist; etching away the nitride layer in asubsequently active area of the silicon wafer as well as in an area ofthe chromium-gold metallic coating; and removing the photoresist.
 8. Aprocess according to claim 1, further comprising an additional processstep of additionally implanting dopants having a conductivity typeopposite to the conductivity type of the silicon wafer so as to create ap-n junction in the subsequently porous silicon layer.
 9. A processaccording to claim 1, wherein the process step of forming the secondcontact includes the forming of an ohmic contact at the side of thesilicon wafer opposite the anode side.
 10. A process according to claim1, wherein the process step of forming the first contact includes theapplication of a gold contact.
 11. A process according to claim 10,wherein the gold contact has a thickness of approximately 120 nm.
 12. Aprocess according to claim 1, wherein the process step of forming thefirst contact includes the application of an indium tin oxide layer. 13.A process according to claim 12, wherein the indium tin oxide layer hasa thickness of approximately 200 nm.
 14. A process according to claim 1,wherein the process step of forming the contacts comprises the followingsteps:applying an at least partially light-transmitting first contact tothe microporous silicon layer; and forming a second contact forestablishing a contact with an area of the silicon wafer below themicroporous silicon layer.
 15. A process according to claim 1, whereinthe anodization region of the silicon wafer is delimited laterally by anacid-resistant masking layer.
 16. A process according to claim 15,wherein the acid-resistant masking layer is light-proof.